(1) Field of the Invention
The present invention relates to an optical amplifier for amplifying wavelength division multiplexed signal light. In particular the invention relates to an optical amplifier for collectively amplifying wavelength division multiplexed signal light which includes optical signals of two wavelength bands.
(2) Description of the Related Art
With conventional long distance light transmission systems, light transmission is performed using light regeneration repeaters which convert optical signals into electric signals to effect re-timing, re-shaping, and regenerating. However recently, with progress in the utilization of optical amplifiers, optical amplifier repeater transmission systems which use optical amplifiers as linear repeaters are being investigated. By replacing a light regeneration repeater with an optical amplifier repeater, the number of parts in the repeater can be greatly reduced, with the prospect of maintaining reliability and greatly reducing costs.
Furthermore, as one method of realizing a large capacity of an optical transmission system, a wavelength division multiplexed (WDM) light transmission system which multiplexes and transmits two or more optical signals having different wavelengths on a single optical transmission path is attracting attention. In a WDM optical amplifier repeater transmission system with the abovementioned optical amplifier repeater transmission system combined with a WDM light transmission system, it is possible to collectively amplify, WDM signal light using an optical amplifier, thus enabling the realization of large capacity and long distance transmission with a simple (economic) construction.
For the wavelength band of the WDM signal light used in the abovementioned conventional optical transmission system, in addition to a so called C band of a 1550 nm band, recently a so called L band of a 1580 nm band is being investigated. Moreover, a WDM optical amplifier repeater transmission system where both are transmitted on a single transmission line (a C/L band WDM optical amplifier repeater transmission system) is being investigated.
As an optical amplifier having an optical amplification band in both the C band and the L band, there is known for example an article “Recent Research Progress in Broadband Optical Fiber Amplifiers” by Yamada et al, Technical Report of IEICE. OCS 97-42, ED 97-132 OPE 97-87 LQE 97-87 (1997-11). The present optical amplifier, as shown in FIG. 23 is of a simple parallel construction where an input WDM signal light is demultiplexed by a demultiplexer into a C band and an L band, and after being respectively amplified in a C band optical amplifying section and an L band optical amplifying section, is multiplexed by a multiplexer and then output
However, since the demultiplexer is provided on the input side of the respective C band and the L band optical amplifying sections, there is a problem that the noise figure deteriorates by the insertion loss portion. As a technique for coping with this, there is known for example an optical amplifier disclosed for example in “Optical Amplifiers and their Applications, Post deadline 2 of '97 published by Lucent Technologies. A schematic construction of this is shown in the block diagram of FIG. 24. With the construction of FIG. 24, a C/L band optical amplifying section having an amplifying band in both the C band and the L band is arranged in the pre-stage of the demultiplexer in the construction of FIG. 23. Hence at least a gain equal to or greater than the insertion loss part of the demultiplexer is ensured by the C/L band optical amplifying section, so that an improvement in the noise figure is achieved.
However, with the above mentioned C/L band optical amplifier, there is the drawback in that there are many cases where the noise figure for the C band optical amplifying section and the noise figure for the L band optical amplifying section are different, so that in general, the optical SN ratio for the optical signal of the L band deteriorates compared to that for the optical signal of the C band. More specifically, in the case where an optical fiber amplifier which uses rare earth element doped fiber is employed in the optical amplifying sections of the respective bands, it is known that the noise figure for the L band optical amplifying section deteriorates by 1 dB.
Furthermore, with the optical amplifier of the construction as shown in FIG. 24, since this has three or more optical fiber amplifier sections, the number of excitation light sources also increases. As a result, the power consumption of the overall optical amplifier increases. In particular, in the case where, as with optical amplifiers used for example as undersea repeaters or the like, there are restrictions from the view point of installation space, power consumption or other factors, it is difficult to realize a construction such as shown in FIG. 24.
Furthermore, It is also important to control the balance of the optical power of the C band and the L band. That is to say, in the case of a setting where the number of optical signals (channel number) included in the respective bands is different, or in the case where this setting is changed during operation, it is necessary to appropriately control the respective operations of the optical amplifying sections for the respective bands to ensure the required transmission characteristics.